A distinct subset of T lymphocytes, called suppressor cells, exists in the immune system to turn down antigen-driven responses, and as a mechanism to maintain tolerance to self-antigens in the periphery. Thus the immune system has a self-regulating mechanism to modulate its reactions especially to cells and tissues of its own. However, there are many situations where the suppres-sion of the immune system is unwanted and may eventually lead to disease.
A number of internal and external factors exist that can cause this suppression:
Stress can have marked effects on the health of fishes. Stress can come in many forms such nutritional stress due to improper diet and feeding schemes; envi-ronmental stress brought about by poor water quality and physical stress atten-dant to handling, crowding or confinement. These types of stress can some-times be unavoidable in intensive fish farming. Prolonged exposure to stress or even very brief stressful experiences can depress certain aspects of the cellular and humoral immune systems consequently lowering resistance to pathogens. Once stress is experienced, a cascade of neuroendocrine events follows that generally leads to elevation of the steroid hormone, cortisol, in circulation. This hormone and other stress hormones (e.g. catecholamines) can reduce the num-ber of circulating leukocytes and antibody-producing cells, and depress mac-rophage activity and distribution of leukocytes in to various body compart-ments.
Aluminum, arsenic, cadmium, chromium, copper, lead, mercury, nickel and zinc are heavy metals that have been shown to lower the non-specific and specific immune response in fish making them more susceptible to various vi-ral and bacterial diseases.
Polychlorinated biphenyls (PCBs) have been reported to have modulating ef-fects on the immune response in fish ranging from no effect reduction of anti-body producing cells and increased susceptibility to disease. Chlorinated di-oxin (TCDD) partially suppressed the mitogenic response in trout. It appears that fishes may not be as sensitive to halogenated aromatic hydrocarbons as do the higher vertebrates, like mice, with regard to their humoral immune system.
Endrin, malathion, methyl bromide, triclorphon, DDT, Bayluscide and tributylin have immunosuppressive properties ranging from reduced lympho-cyte number and phagocyte activity to necrosis of the thymus.
Oxytetraxycline, the antibiotic most used by fish culturist in treatment of bacte-rial disease, has been long known to be immunosuppressive in fish, reducing the numbers of antibody-producing cells. Regardless of the mode of adminis-tration (injection, feeding or bath) an immunosuppressive effect is evident. However, oxolinic acid, a more recent addition to the list of drugs for treating fish bacterial disease, was found to not have immunosuppressive properties when used at the therapeutic levels recommended.